Heat exchange tube for use in a boiler and boiler incorporating the same

ABSTRACT

A gas- or oil-fired boiler incorporating a plurality of heat exchange tubes disposed above the gas or oil burner, the hot flue gases from the burner operatively flowing through the tubes which are in heat exchange relationship with the water to be heated. Each heat exchange tube is formed of two members of L-shape in cross-section, the members which are identical being so interconnected in a fluid-tight manner that the tube is of substantially rectangular cross-section with one of the pairs of opposed side walls of the tube presenting inwardly directed projections in the form of abutting segments through the tube and to cause turbulence in the gases, thereby to improve the heat exchange efficiency of the boiler. The abutting projections also serve to prevent collapse of the tube caused by external water pressure acting thereon, the members constituting the tube being relatively thin sheet metal stampings.

[451 Sept. 4, 1973 Unite States Patent [1 1 Keith 3,215,124 11/1965 Shoemaker.... ...............,....122/165 1 HEAT EXCHANGE TUBE FOR USE IN A BOILER AND BOILER INCORPORATING THE SAME Primary Examiner-Kenneth W. Sprague Att0rneyJefferson Ehrlich [75] Inventor:

Clarence Henry Keith, Toronto, Ontario, Canada [57] ABSTRACT A gasor oil-fired boiler incorporating a plurality of heat exchange tubes disposed above the gas or oil burner, the hot flue gases from the burner operatively flowing through the tubes which are in heat exchange relationship with the water to be heated. Each heat exchange tube is formed of two members of L-shape in cross-section, the members which are identical being so interconnected in a fluid-tight manner that the tube is of substantially rectangular cross-section with one of 0 m mm m 67 w 2 e 2 N IF l u m 7 m s w m n i n .m nu m .0, n" Y mm m 00 u ANNZ "m m N 0 .e m a .mw sm A FA UI 1. ll. 1.] 3 2 2 7 B2 55 l 1 ll [58] Field of Search.................... 122/136 C, 155 A, 122/155 C, 156, 165, 367

the pairs of opposed side walls of the tube presenting inwardly directed projections in the form of abutting segments through the tube and to cause turbulence in [56] References Cited UNITED STATES PATENTS the gases, thereby to improve the heat exchange efficiency of the boiler. The abutting projections also serve to prevent collapse of the tube caused by external 1,093,429 4/1914 Jacobs et al. 122/136 2,906,250

55 66 II 22 22 II 9/1959 Clarkson.... 5/1931 Jerome..

water pressure acting thereon, the: members constituting the tube being relatively thin sheet metal stampings.

10/1959 Olsen...

15 Claims, 4 Drawing Figures PATENTEDSEP 4 I973 SHEH 1 OF 2 FIG 2 FIG] INVENTOR CLARENCE H, KEITH ATTORNEYS PATENTED SEP 4 I973 SHEET 2 0F 2 INVENTOR. CLARENCE H KEHH ATTORNEYS HEAT EXCHANGE TUBE FOR USE IN A BOILER AND BOILER INCORPORATING THE SAME This invention is concerned with heat exchange tubes for use in boilers, and with boilers, of both the gasand oil-fired types, incorporating such tubes.

To ensure maximum operating efficiency of such boilers it is, of course, necessary for the maximum possible heat transfer operatively to occur through the side walls of the heat exchange tube or tubes from the hot flue gases to the water to be heated in the boiler. For this reason, it is desirable for the flow of hot flue gases through the heat exchange tube or tubes, where the hot flue gases operatively pass through said tube or tubes, to be retarded and for turbulence to be created therein. It has hitherto been proposed to provide heat exchange tubes in which this desirable result is achieved, reference in this connection being made to, for example, US. Pat No. 514,338 which issued on Feb. 6th, 1894 to O.M. Row. In the arrangements of Row, each heat exchange tube is provided with indentations or depressions which serve to retard the flow of hot flue gases through the tube and to create turbulence in the hot flue gases, thereby to increase the efficiency of heat transfer through the side walls of the tube.

While the heat exchange tubes disclosed by Row serve effectively to increase the efficiency of heat transfer through the side walls of the tubes in the above-described manner, it is to be noted that the heat exchange tubes of Row are all of circular cross-section, the manner in which the indentations or depressions are formed in the tubes being specific to tubes of this cross-sectional form in that these indentations or depressions are formed at intervals along the length of the tube, with the indentations being alternately formed at right angles to one another. For certain forms of boilers it may, however, be preferable for the heat exchange tubes to be of rectangular cross-sectional form, rather than of circular cross-sectional form, and it is a primary object of the present invention to provide a heat exchange tube, and a boiler incorporating such a tube or tubes, in which the tube or tubes are ofrectangular cross-sectional form but in which the efficiency of heat transfer through the side walls of the tube or tubes is increased relative to the efficiency of heat transfer through the side walls of the tubes in boilers having heat exchange tubes of rectangular cross-sectional form as hitherto known.

It is also a primary object of the present invention to provide a heat exchange tube, and a boiler incorporating such a tube or tubes, in which the tube of rectangular cross sectional form is substantially prevented from operatively collapsing as a result of external fluid pressure acting on the tube, this problem being substantially non-existent in the case of tubes as disclosed by Row since it is well known that tubes of circular crosssection have a high resistance to collapse under the in- 2 enting inwardly directed projections with the projections presented by said opposed side walls of the tube being in abutting contact.

In a hot water or steam boiler according to theinvention, there is incorporated at least one heat exchange tube according to the invention, the boiler comprising a burner, a first passage for the flow of hot flue gases from the burner to a flue gas discharge, and a second passage interconnecting a water inlet with a hot water or steam outlet. At least a portion of the first passage is in heat exchange relationship with at least a portion of the second passage, with said portion of one of said first and second passages being constituted by said at least one heat exchange tube.

In order that the invention may be more clearly understood and more readily carried into effect, the same will now, by way of example, be more fully described with reference to the accompanying drawings in which:

FIG. 1 is a sectional view, on the line 1--1 of FIG. 2, of a gas-fired boiler according to a preferred embodiment of the invention;

FIG. 2 is a sectional side view, on the line 2--2 of FIG. 1, of the gas-fired boiler illustrated therein;

FIG. 3 is a sectional view, on the line 3--3 of FIG. 4, of an oil-fired boiler according to a preferred embodiment of the invention; and

FIG. 4 is a sectional side view, on the line 4-4 of FIG. 3, of the oil-fired boiler illustrated therein.

Referring to all the views of the drawings, in which like reference numerals are used to denote like parts, 10 denotes generally a hot water or steam boiler which comprises a boiler shell 11 of sheet metal construction, and a sheet metal outer casing 12 in which the sides and top of the boiler shell 1 l are encased, the inner surfaces of the casing 12 being covered by layers 13 of suitable heat insulation material. For clarity, the outer casing 12 and the layers 13 have been omitted from FIGS. 3 and 4 of the drawings. The boiler 10 also comprises a burner 14 from which there extends a first passage 15 for the flow of hot flue gases in the direction of the arrows A, i.e., the full line arrows, in FIGS. 2 and 4, the

, passage. 15 extending from the burner 14 to a flue gas discharge which is denoted by the reference numeral 16 in FIGS. 1 and 2 but which, for simplicity, is omitted from FIGS. 3 and 4. A second passage 17 interconnects a water inlet 18 with a hot water or steam outlet 19, for locations of the water inlet 18 and. of the hot water or steam outlet 19 which would not, of course, appear in sectional side views on the line 2--2 of FIG. 1 and on the line 4-4 of FIG. 3, respectively, being however indicated in chain-dotted lines (arrows B) in FIGS. 2 and 4, respectively. Means (not shown), which may be of conventional form and which senses the temperature of the water flow through the second passage 17, when the boiler 10 is in use, is incorporated in the boiler, this means causing operation of a circulator (not shown), which serves to circulate the water through the passage 17 and causes the circulator to cease when the water temperature is below a predetermined value.

This means, which senses the temperature of the water flow through the second passage 17 when the boiler 10 is in use, also so influences the control of the fuel flow to the burner 14 that the supply of fuel to the burner 14 is shut-off when the temperature of the water within the passage 17 attains a predetermined value. A portion of the first passage 15 is in heat exchange relationship with a portion of the second passage 17, said portion of the first passage being constituted by a plurality of heat exchange tubes each of which is denoted generally by the reference numeral 20. It is to be understood that, in alternative embodiments (not shown) of the invention, the tubes may constitute substantially the entirety of the first passage 15, and that the tubes 20 maybe in heat exchange relationship with substantially the entirety of the second passage 17.

Each tube 20 is of substantially rectangular cross sectional form, i.e., is of substantially rectangular cross section in a direction at right angles to the general direction of flow of hot flue gases therethrough as indicated by the arrows A, the tube 20 having an open end 21 for the admission of hot flue gases thereto and an open end 22 for the discharge of the hot flue gases therefrom. The tube 20 thus comprises a first pair of opposed side walls 23 and a second pair of opposed side walls 24, the opposed side walls 23 of the tube 20 presenting inwardly directed projections 25 which are each in the form of a segment of a sphere. The projections 25 presented by one of the side walls 23 of the tube 20 are each directly in alignment with, and in abutting contact with, one of the projections 25 presented by the other of the side walls 23 of the tube 20, the projections 25 presented by each of the side walls 23 being disposed in a plurality of equispaced rows 26 and 27 disposed parallel to the longitudinal axis of the tube 20, with the projections 25 of alternate rows, i.e., the rows 26 or the rows 27 being in alignment, and with the projections 25 of adjacent rows, i.e., one of the rows 26 and an adjacent one of the rows 27, being in staggered relationship.

With reference to the preferred embodiment of the invention illustrated in FIGS. 1 and 2, there is mounted within each of the three centrally disposed tubes 20, as viewed in FIG. 1, a baffle plate 28 which operatively tends to prevent the hot flue gases flowing upwardly through the appropriate tube 20 from converging towards the centre of the tube 20, as viewed in FIG. 2, as they pass through the portion of the tube 20 adjacent to the open end 22 thereof. Each baffle plate 28 is of substantially inverted V-shape with the free end portions 29 of the limbs 30 depending from the remaining portions of the limbs 30 in substantially vertical planes. It is to be emphasized, of course, that baffle plates 28 may be mounted in any or all of the tubes 20.

Each tube 20 is, as is most clearly shown in FIG. 3, constituted by two identical members 31, each member 31 being of substantially L-shape in cross-section and comprising a first limb 32 which constitutes one of the side walls 23 of the tube 20 and a second limb 33 which constitutes one of the side walls 24 of the tube 20. The first limb 32 presents a first longitudinal edge 34 remote from the second limb 33 and the second limb 33 presents a second longitudinal edge 35 remote from the first limb 32, the first longitudinal edge 34 of each member 31 being connected in a fluid-tight manner, as, for example, by welding, to the second longitudinal edge 35 of the other member 31 constituting the tube 20. While the above-described manner by which each tube 20 is formed of two identical members 31 has been described with reference to FIG. 3, it is to be understood that each tube 20, in the embodiment of the invention illustrated in FIGS. 1 and 2, is formed in a corresponding manner. The members 31 are each formed of a relatively thin sheet metal stamping so that concave depressions 36, each of which is in the form of a segment of a sphere, are thereby provided in the outer face of the side walls 23, each depression 36 being, of course, in alignment with one of the inwardly directed projections 25. As will be appreciated, each tube 20, and hence also the associated boiler 10 within which each tube 20 is incorporated, is thus of simple and inexpensive construction.

With particular reference to the embodiment of the invention illustrated in FIGS. 1 and 2 of the drawings, the boiler 10 is a gas-fired boiler with the burner 14 being constituted by a plurality of gas burnertubes 37 to which gas is operatively supplied through a gas flow control device illustrated diagrammatically and denoted by the reference numeral 38, the tubes 37 being arranged in side-by-side relationship and being, if desired, of conventional form. The heat exchange tubes 20 are disposed in spaced side-by-side relationship above the gas burner tubes 37 with the axis of the flue gas passage of each tube 20 substantially vertical so that the direction of flow of the hot flue gases through each tube 20 is in a generally upward vertical direction, as indicated by the appropriate arrows A. The second passage 17, along which the water operatively being heated in the boiler 10 flows, is in effect of reflexly bent form since the velocity of the water entering the passage 17 from the water inlet 18 is such that the water is caused to flow past the tubes 20 and then to be redirected back past the tubes 20 to the hot water or steam outlet 19, the axis of the portions of the second passage 17 with which the tubes 20 are in heat exchange relationship thus being approximately horizontal as indicated by the appropriate arrows B, i.e., the broken line arrows, so that, in operation, the direction of flow of the water past the heat exchange tubes 20 is approximately at right angles to the direction of flow of the hot flue gases through the tubes 20. It is, of course, to be understood that, in alternative forms (not shown) of the embodiment of the invention shown in FIGS. 1 and 2, the water inlet 18 and the hot water or steam outlet 19 may be so positioned that the water, in operatively flowing along the second passage 17, flows past the tubes 20 only once so that the passage 17 is not of reflexly bent form, thelongitudinal axis of the portion of the second passage 17 with which the tubes 20 are in heat exchange relationship again being substantially horizontal and the operative direction of flow of water past the tubes 20 again therefore being substantially at right angles to the direction of flow of hot flue gases through the tubes 20.

39 denotes generally a flue gas outlet structure which may be of conventional form and which presents the flue gas discharge 16.

With particular reference to the embodiment of the invention illustrated in FIGS. 3 and 4 of the drawings, the boiler 10 is an oil-fired boiler with the burner 14 thus being constituted by an oil burner 40 which may be of conventional form and which incorporates a burner nozzle 41 disposed within a well 42 of refractory material. The heat exchange tubes 20 are disposed in spaced side-by-side relationship above the burner 40 with the axis of the flue gas passage of each tube 20 substantially horizontal so that the direction of flow of the hot flue gases through each tube 20 is in a generally horizontal direction as indicated by the appropriate arrows A, the axis of the portion of the second passage 17 with which the tubes 20 are in heat exchange relationship being substantially vertical as indicated by the appropriate arrows B so that, in operation, the direction of flow of the water past the heat exchange tubes 20 is substantially zig-zaggedly at right angles to the direction of flow of the hot flue gases through the tubes 20, as in the case of the embodiment illustrated in FIGS. 1 and 2 of the drawings. Between the burner 40 and the tubes 20, the passage 15 incorporates a deflector plate 43, see FIG 3, which is disposed above the burner 40 and which serves to redirect the hot flue gases back towards the burner 40 as indicated by the appropriate arrows A. In this manner, any unburned oil, or incompletely combusted particles in the hot flue gases, are reheated as the hot flue gases are redirected back towards the burner 40 with a resultant increase in the efficiency of combustion, and a resultant-reduction in the degree of pollution when the flue gases pass to the atmosphere by way of the flue gas discharge (not shown in FIGS. 3 and 4).

The portion of the second passage 17 between the water inlet 18 and the portion of said passage 17 with which the tubes 20 are in heat exchange relationship is, in the preferred embodiment of the invention illustrated in FIGS. 3 and 4, of bifurcated form and comprises a part 44 with which the water inlet 18 is in communication, spaced parts 45 which are disposed one on either side of the deflector plate 43 and which are in communication with the ends of the part 44; and a part 46 with which the parts 45 are in communication, the part 46 and also the parts 45 adjacent to the part 46 being upwardly open to permit the water to flow therefrom in the directions of the appropriate arrows B.

Referring again to all the views of the drawings, the hot flue gases created, when the boiler is in operation, by the combustion of gas at the tubes 37 (FIGS. 1 and 2) or by the combustion of oil at the burner nozzle 41 (FIGS. 3 and 4), pass through the first passage in the direction of the arrows A to the flue gas discharge 16, the gases in flowing through the passage 15 passing, of course, through the heat exchange tubes through the side walls of which heat is transferred to the water flowing through the second passage 17 from the water inlet 18 to the hot water or steam outlet 19. This water flowing through the passage 17 is thus heated.

As will be noted particularly from FIGS. 2 and 4 of the drawings, the hot flue gases in passing through the heat exchange tubes 20 are retarded and turbulence is created therein, this retardation and turbulence, which will be apparent from the appropriate arrows A, being caused by the inwardly directed projections 25. Likewise, the concave depressions 36 in the outer faces of the side walls 23 of the tubes 20 serve to retard and create turbulence in the water flow through the second passage 17 past the tubes 20. The efficiency of heat transfer through the side walls of the tubes 20 between the hot flue gases and the water is thus substantially increased, while the abutting contact between the projections 25 presented by the opposed side walls 23 of each tube 20 serves substantially to prevent collapsing of the tube 20 as a result of the water pressure within the second passage 17, this water pressure being, in many cases of the order of 70 p.s.i.

While in the preferred embodiments of the invention hereinbefore described with reference to the accompanying drawings, each boiler 10 incorporates a plurality of heat exchange tubes 20, the gas-tired boiler illustrated in FIGS. 1 and 2 incorporating five such tubes 20 and the oil-fired boiler illustrated in FIGS. 3 and 4 incorporating four such tubes 20, it is to be understood that, in alternative embodiments (not shown) of the invention, the boiler 10 may incorporate a minimum of one heat exchange tube 20, although a boiler 10 incorporating only one heat exchange tube 20 would, of course, be less efficient than a boiler 10 incorporating a plurality of such tubes 20. Furthermore, in alternative embodiments (not shown) of the invention, the opposed side walls 23 of each tube 20 may each present only a single projection 25, although the form of the tubes 20 as hereinbefore described with reference to the accompanying drawings in which each side wall 23 of each tube 20 presents a plurality of such inwardly directed projections 25 is preferred. In alternative embodiments (not shown) of the invention, there may, moreover, be one or more inwardly directed projections presented by each side wall 224 of each heat exchange tube 20, these projections presented by the side walls 24 corresponding to the projections 25 hereinbefore described with reference to the drawings, although, where the opposed side walls 24 of each tube 20 present inwardly directed projections corresponding to the inwardly directed projections 25 presented by the opposed side walls 23, the disposition of the projections is, of course, different from that shown in the ac companying drawings in order to permit the projections presented by the opposed side walls 24 to be in abutting contact, as well as to permit the projections 25 presented by the opposed side walls 23 to be in abutting contact.

It is also to be understood that, while in the preferred embodiments of the invention hereinbeforedescribed with reference to the accompanying drawings, the heat exchange tubes 20 constitute a portion of the first passage 15, i.e., the passage extending from the burner 14 to the flue gas discharge 16 and along which the hot flue gases operatively flow, the tubes 20 may in alternative embodiments (not shown) of the invention constitute the portion of the second passage 17, i.e., the passage interconnecting the water inlet 18 and the hot water or steam outlet 19 and along which the water being heated in the boiler 10 operatively flows, which is in heat exchange relationship with the first passage 15.

What is claimed is:

l. A heat exchange tube for use in a boiler for transferring heat from one fluid to another fluid the tube being of open-ended form and being of substantially rectangular cross-section, said tube having at least one pair of opposed side walls for separating one of said fluids from the other and presenting inwardly directed projections, the projections presented by said opposed side walls of the tube being in abutting contact or almost in contact, both fluids being rendered turbulent as they encounter said projections.

2. A heat exchange tube according to claim I, wherein each projection is in the form of a segment of a sphere. I

3. A heat exchange tube according to claim I, wherein each of the opposed side walls of the tube presents a plurality of said inwardly directed projections disposed in a plurality of equi-spaced rows, the projections of alternate rows thereof being in alignment. and the projections of adjacent rows thereof being in stagtglertcald relationship to improve the turbulence of both 4. A heat exchange tube according to claim 1, wherein the projections presented by one of each pair of opposed side walls of the tube are directly in alignment with the projections presented by the other of said opposed side walls of the pair so that the joinder of the projections forms protection for the tube against collapse from external fluid pressure and also aids in producing fluid turbulence.

5. A heat exchange tube according to claim 3, wherein each projection presented by each pair of the opposed side walls of the tube, is directly in alignment with each of the projections presented by the other of said opposed side walls of the pair.

6. A heat exchange tube according to claim 3, wherein the tube is constituted by two identical members, each of which is of substantially L-shape in crosssection and presents a first longitudinal edge and a second longitudinal edge, the first longitudinal edge of each member being connected in a fluid-tight manner to the second longitudinal edge of the other member.

7. A heat exchange tube according to claim 3, wherein each pair of opposed side walls of the tube are provided in the outer faces thereof with concave depressions which are in alignment with the projections presented by said opposed side walls.

8. A hot water or steam boiler comprising a burner adjacent the base of the boiler, a first passage for the flow of hot flue gases substantially vertically upwardly from the burner to a flue gas discharge port, and a second passage interconnecting a water inlet with a hot water or steam outlet, at least a portion of said first passage being in heat exchange relationship with at least a portion of said second passage, with said portion of one of said first and second passages being constituted by a heat exchange arrangement which is composed of a vertical tubular structure of substantially rectangular cross-section and having at least one pair of opposed side walls presenting inwardly directed projections with the projections presented by said opposed side walls of the tube being in abutting contact or almost in contact, for rendering both the flue gases and the water turbulent.

9. A boiler according to claim 8, wherein said tubular structure is constituted by two identical members each of which is substantially L-shape in cross-section and presents a first longitudinal edge and a second longitudinal edge, the first longitudinal edge of each member being connected in a fluid-tight manner to the second longitudinal edge of the outer member.

10. A boiler according to claim 9, wherein said portion of one of said first and second passages which is constituted by said tubular structure is constituted by said portion of the first passage and is disposed, above the burner, with the axis of the first passage thereof substantially vertical, said portion of the second passage being disposed with the axis thereof substantially horizontal.

11. A boiler according to claim 9, wherein said portion of one of said first and second passages which is constituted by said tubular structure is constituted by said portion of the first passage and is disposed, above the burner, with the axis of the first passage thereof substantially horizontal, said portion of the second passage being disposed with the axis thereof substantially vertical.

12. A boiler according to claim 9, wherein said portion of one of said first and second passages which is constituted by said tubular structure is constituted by said portion of the first passage, and wherein the burner is an oil burner and the first passage incorporates, between the oil burner and the portion of the first passage constituted by said tubular structure, a deflector plate which is shaped and disposed above the oil burner for redirecting hot gases toward the oil burner, said portion of the first passage which is constituted by said tubular structure being disposed, above the burner, with the axis of the first passage thereof substantially horizontal, and said portion of the second passage being disposed with the axis thereof substantially vertical.

13. A boiler comprising a burner at the base of the boiler for producing in the boiler a first fluid which is to be heated and transmitted through a first passage, means for furnishing a second fluid to be heated and transmitted through a second passage of the boiler, a heat exchanger coupling said first and second passages, said heat exchanger having a plurality of substantially parallel vertical tubes of rectangular cross-section when viewed substantially in the directions in which the respective fluids are transmitted, the opposing walls of each tube having respective projections which abut each other so as to structurally reinforce the tube against the external pressure of the second fluid as the first fluid is transmitted through the tube and so as to create turbulence for the flow of both fluids.

14. A boiler in accordance with claim 13, in which the abutting projections of each tube are spherical segments which are tangential to each other.

15. A boiler according to claim 14, in which the respective projections within each tube are staggered with respect to the respective projections of each adjacent tube so as to further increase the turbulence in the flow of the second fluid.

i ll l I l 

1. A heat exchange tube for use in a boiler for transferring heat from one fluid to another fluid the tube being of open-ended form and being of substantially rectangular cross-section, said tube having at least one pair of opposed side walls for separating one of said fluids from the other and presenting inwardly directed projections, the projections presented by said opposed side walls of the tube being in abutting contact or almost in contact, both fluids being rendered turbulent as they encounter said projections.
 2. A heat exchange tube according to claim 1, wherein each projection is in the form of a segment of a sphere.
 3. A heat exchange tube according to claim 1, wherein each of the opposed side walls of the tube presents a plurality of said inwardly directed projections disposed in a plurality of equi-spaced rows, the projections of alternate rows thereof being in alignment, and the projections of adjacent rows thereof being in staggered relationship to improve the turbulence of both fluids.
 4. A heat exchange tube according to claim 1, wherein the projections presented by one of each pair of opposed side walls of the tube are directly in alignment with the projections presented by the other of said opposed side walls of the pair so that the joinder of the projections forms protection for the tube against collapse from external fluid pressure and also aids in producing fluid turbulence.
 5. A heat exchange tube according to claim 3, wherein each projection presented by each pair of the opposed side walls of the tube, is directly in alignment with each of the projections presented by the other of said opposed side walls of the pair.
 6. A heat exchange tube according to claim 3, wherein the tube is constituted by two identical members, each of which is of substantially L-shape in cross-section and presents a first longitudinal edge and a second longitudinal edge, the first longitudinal edge of each member being connected in a fluid-tight manner to the second longitudinal edge of the other member.
 7. A heat exchange tube according to claim 3, wherein each pair of opposed side walls of the tube are provided in the outer faces thereof with concave depressions which are in alignment with the projections presented by said opposed side walls.
 8. A hot water or steam boiler comprising a burner adjaCent the base of the boiler, a first passage for the flow of hot flue gases substantially vertically upwardly from the burner to a flue gas discharge port, and a second passage interconnecting a water inlet with a hot water or steam outlet, at least a portion of said first passage being in heat exchange relationship with at least a portion of said second passage, with said portion of one of said first and second passages being constituted by a heat exchange arrangement which is composed of a vertical tubular structure of substantially rectangular cross-section and having at least one pair of opposed side walls presenting inwardly directed projections with the projections presented by said opposed side walls of the tube being in abutting contact or almost in contact, for rendering both the flue gases and the water turbulent.
 9. A boiler according to claim 8, wherein said tubular structure is constituted by two identical members each of which is substantially L-shape in cross-section and presents a first longitudinal edge and a second longitudinal edge, the first longitudinal edge of each member being connected in a fluid-tight manner to the second longitudinal edge of the outer member.
 10. A boiler according to claim 9, wherein said portion of one of said first and second passages which is constituted by said tubular structure is constituted by said portion of the first passage and is disposed, above the burner, with the axis of the first passage thereof substantially vertical, said portion of the second passage being disposed with the axis thereof substantially horizontal.
 11. A boiler according to claim 9, wherein said portion of one of said first and second passages which is constituted by said tubular structure is constituted by said portion of the first passage and is disposed, above the burner, with the axis of the first passage thereof substantially horizontal, said portion of the second passage being disposed with the axis thereof substantially vertical.
 12. A boiler according to claim 9, wherein said portion of one of said first and second passages which is constituted by said tubular structure is constituted by said portion of the first passage, and wherein the burner is an oil burner and the first passage incorporates, between the oil burner and the portion of the first passage constituted by said tubular structure, a deflector plate which is shaped and disposed above the oil burner for redirecting hot gases toward the oil burner, said portion of the first passage which is constituted by said tubular structure being disposed, above the burner, with the axis of the first passage thereof substantially horizontal, and said portion of the second passage being disposed with the axis thereof substantially vertical.
 13. A boiler comprising a burner at the base of the boiler for producing in the boiler a first fluid which is to be heated and transmitted through a first passage, means for furnishing a second fluid to be heated and transmitted through a second passage of the boiler, a heat exchanger coupling said first and second passages, said heat exchanger having a plurality of substantially parallel vertical tubes of rectangular cross-section when viewed substantially in the directions in which the respective fluids are transmitted, the opposing walls of each tube having respective projections which abut each other so as to structurally reinforce the tube against the external pressure of the second fluid as the first fluid is transmitted through the tube and so as to create turbulence for the flow of both fluids.
 14. A boiler in accordance with claim 13, in which the abutting projections of each tube are spherical segments which are tangential to each other.
 15. A boiler according to claim 14, in which the respective projections within each tube are staggered with respect to the respective projections of each adjacent tube so as to further increase the turbulence in the flow of the second fluid. 